This project investigated yeast selection’s effects on the volatile phenols associated with wines made from smoke-exposed Chenin blanc juice.
Introduction
Several wine-producing countries, including Australia, Chile, Greece, France, Italy, Portugal, Spain, South Africa and the United States of America, have been affected by fire or smoke damage to vineyards.1
Some of these fires are so devastating that entire vineyards and regions are destroyed. To date, Australia and USA suffered substantial losses in revenue due to fire and smoke damage to vineyards and wines.2,3 Wines produced from smoke-exposed grapes can have unpleasant, burnt, medicinal, smoky and ashy characteristics, collectively known as “smoke taint”.1,4
The compounds usually associated with this taint are known as volatile phenols. Guaiacol, 4-ethyl guaiacol, 4-methylguaiacol and 4-ethylphenol are the main volatile phenols used as chemical ‘markers’ for smoke-tainted wine.5
These volatile phenols and their precursors accumulate in grapes and leaves after smoke exposure and are then released into the wines during winemaking.
The yeasts responsible for transforming grape must into wine through alcoholic fermentation contribute to flavour and quality through producing and modifying metabolites, including volatile phenols.
Commercial yeasts have differing abilities to affect the volatile phenols in wine.
Consequently, this study investigated the levels of volatile phenols in Chenin blanc wines fermented with a range of different yeasts to determine if some are better suited for producing wines from smoke-affected grapes by reducing the quantity of smoky phenols or by masking it.
This article summarises the scientific article published in the Fermentation journal.6
Materials and methods
A selection of 12 commercial yeasts was evaluated in laboratory-scale (200 mL) fermentation trials using smoke-exposed Chenin blanc juice.
This was followed by a small-scale (14 L) wine production trial with smoked and unsmoked Chenin blanc juice with a selection of three yeasts, namely QA23, VIN 13 and X16.
Wines produced from the smoke-exposed juice will be referred to as smoked wines.
The concentrations of the volatile phenols were determined, and the wines were also subjected to sensory evaluation.
Results and discussion
The guaiacol and 4-methylguaiacol levels varied in the laboratory-scale Chenin blanc wines produced with the different yeast strains (Figure 1 and 2).
Wines made with the yeast QA23 contained the highest guaiacol and 4-methylguaiacol levels, while wines fermented with VIN 13 and X16 contained the lowest levels.
All the other treatments ranged somewhere in between, but not in the same order for the two compounds. QA23 is known to have β-glucosidase activity, a desired trait that helps release varietal aroma compounds.
In this case, it can also increase the volatile phenol levels in wine by liberating them from their glycoconjugate form.
FIGURE 1. Guaiacol levels in laboratory-scale Chenin blanc wines produced from smoke-exposed juice fermented with selected commercial yeasts or spontaneous alcoholic fermentation (Spont. Ferment).
FIGURE 2. 4-Methylguaiacol levels in laboratory-scale Chenin blanc wines produced from smoke-exposed juice fermented with selected commercial yeasts or spontaneous alcoholic fermentation (Spont. Ferment).
In the small-scale trial, wines produced from unsmoked and smoked juice fermented with QA23, VIN 13 and X16 were compared.
The wines produced from the smoke-exposed juice contained notably higher levels of guaiacol and 4-methylguaiacol than the unsmoked treatments (Figure 3).
As with the laboratory-scale trial, smoke-exposed juice fermented with QA23 had the highest guaiacol levels and those fermented with VIN 13 the lowest.
However, X16 also produced similar high guaiacol levels as QA23 in the Chenin blanc wines produced from smoke-exposed juice.
The 4-methylguaiacol levels in all the smoked wines were well below the detection threshold and relatively similar for all three yeasts.
The volatile phenol levels in these experiments were not as high as those found in some commercial wines produced from grapes exposed to wild or vineyard fires. Still, the sensory results showed that the volatile phenols affected wine flavour even at low levels.
FIGURE 3. Guaiacol and 4-methylguaiacol levels in Chenin blanc wines produced from smoke-exposed juice fermented with three commercial yeasts (VIN 13, QA23 and X16). Smoke treatment is abbreviated as ST.
The odour detection threshold of guaiacol is 23 µg/L and is associated with burnt, smoky, toasty, phenolic characteristics,7 while it is 21 µg/L for 4-methylguaiacol, perceived as sweet-spicy, phenolic and leathery.8
Although the volatile phenols levels in this study were considerably below the odour detection threshold, the sensory panel could distinguish between the smoked and unsmoked treatments and preferred the unsmoked wines.
In this trial, none of the three yeast strains effectively masked the negative flavours (smoky, sulphur, vegetative and cooked vegetable characteristics) associated with smoke exposure through the production of varietal or yeast-derived flavour compounds.
However, the flavour profiles of the smoked wines produced with the selected yeast were still slightly different from each other.
This indicates that even though yeast selection may not eliminate the perception of smoke taint in wines, selecting a low guaiacol-producing strain may minimise volatile phenol levels.
Therefore, yeast choice is very important when producing wines from smoke-exposed grapes. Yeast suppliers can help with this choice.
Conclusions
Yeast selection can affect the volatile phenol levels and flavour profiles of wines.
Yeast selection may therefore be a tool to minimise the levels of volatile phenols in wines, but it may not completely mask the negative flavours associated with volatile phenols.
Volatile phenols can have a negative effect on wine flavour, even at levels far below their odour detection threshold.
Another strategy is to maximise volatile phenols in wines and then remove them with fining agents as suggested by some suppliers of oenological products.
In such a case, a yeast with high guaiacol-producing capabilities is desired.
Winemakers should contact their yeast suppliers when working with grapes exposed to smoke and ask them to assist with selecting low or high guaiacol-producing yeast strains for fermentation, depending on their chosen strategy.
Abstract
Wines produced from smoke-exposed grapes can exhibit unpleasant smoky, burnt or ashtray characteristics.
Chenin blanc juice was exposed to smoke and fermented with various commercial wine yeasts. The levels of volatile phenols were determined, and the wines were also subjected to sensory evaluations.
The commercial yeast strains produced wines containing variable levels of guaiacol and 4-methylguaiacol.
Wines made with QA23 contained the highest guaiacol levels, while those produced with VIN 13 had the lowest levels.
Wines produced from smoke-exposed juice contained significantly higher levels of guaiacol and 4-methylguaiacol than unsmoked wines.
A sensory panel could distinguish between wines produced from smoked and unsmoked juice and preferred the unsmoked wines.
Wines from smoked juice were associated with smoky, sulphur, vegetative and cooked vegetable characteristics.
Yeast selection can minimise/mask or maximise the volatile phenol levels in wines.
Winemakers should ask their suppliers for assistance with yeast selection and should use strains that produce low or high guaiacol and 4-methylguaiacol levels depending on their chosen volatile phenol management strategies.
Acknowledgements
The authors thank the ARC and Winetech for funding. All the students and interns who assisted with this project are thanked for their technical assistance. Collaborators from Stellenbosch University are thanked for their contributions.
References
- Mirabelli-Montan, Y.A., Marangon, M., Graça, A., Mayr Marangon, C.M. & Wilkinson, K.L. 2021. Techniques for mitigating the effects of smoke taint while maintaining quality in wine production: A review. Molecules 26(6), 1672.
- Brodison, K. 2013. Effect of smoke in grape and wine production. Department of Agriculture and Food, Western Australia, Perth. Bulletin 4847.
- Congressional Research Service. Wildfire statistics. Available online: https://fas.org/sgp/crs/misc/IF10244.pdf (accessed on 1 June 2021).
- Kennison, K.R., Wilkinson, K.L., Williams, H.G., Smith, J.H. & Gibberd, M.R., 2007. Smoke-derived taint in wine: Effect of postharvest smoke exposure of grapes on the chemical composition and sensory characteristics of wine. Journal of Agricultural and Food Chemistry, 55,10897-10901.
- Ristic, R., Osidacz, P., Pinchbeck, K.A., Hayasaka, Y., Fudge, A.L. & Wilkinson, K.L., 2011. The effect of winemaking techniques on the intensity of smoke taint in wine. Australian Journal of Grape and Wine Research, 17, S29-S40.
- Du Plessis, H.W., Hoff, J.W., Mokwena, L., van der Rijst, M., & Jolly, N.P. 2021. Impact of yeast selection on volatile phenol levels of wines produced from smoked-exposed juice. Fermentation, 7(4), 240.
- Parker, M., Osidacz, P., Baldock, G., Hayasaka, Y., Black, C., Pardon, K., Jeffery, D., Geue, J., Herderich, M. & Francis, I. 2012. Contribution of several volatile phenols and their glycoconjugates to smoke-related sensory properties of red wine. Journal of Agricultural and Food Chemistry, 60, 2629-2637.
- Czerny, M., Christlbauer, M., Fischer, A., Granvogl, M., Hammer, M., Hartl, C. & Hernandez, N. 2008. Re-investigation on odour thresholds of key food aroma compounds and development of an aroma language based on odour qualities of defined aqueous odorant solutions. European Food Research and Technology, 228, 265-273.
– For more information, please contact Heinrich du Plessis at dplessishe@arc.agric.za.
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